Question

why bending of light ray takes place during refraction

Answer 1

it is so because the light changes the medium of traveling example from air to water

Answer 2

While travelling from point A to B, light always takes the path of least time, not the least distance. This means that if the points A and B are in the same medium, the path of least time and least distance will be the same: the straight line joining the points. But if the points are in two different media having two different refractive indices, the path of least time will not be a straight line. That's because the speed of light in different media are different, being less in one and more in other. ( This could be because photons are absorbed and reemitted by atoms in the material, introducing delays)

Thus, to spend minimum time in travelling, a specific ratio of distance traversed in each media is followed by the light ray. So that implies that more distance would be travelled in the medium having less refractive index, and vice versa.

Above is a digram showing the same. If calculated, path 1 will require more time than path 2; provided the refractive index of material 2 is greater than that of material 1. So the light ray bends.

Hope this helps!
Thanks!